Substrate specificity and stereoselectivity of horse liver alcohol dehydrogenase. Kinetic evaluation of binding and activation parameters controlling the catalytic cycles of unbranched, acyclic secondary alcohols and ketones as substrates of the native and active-site-specific Co(II)-substituted enzyme

Enhanced active-site electric field accelerates enzyme catalysis

The design and improvement of enzymes based on physical principles remain challenging. Here we demonstrate that the principle of electrostatic catalysis can be leveraged to substantially improve a natural enzyme's activity. We enhanced the active-site electric field in horse liver alcohol dehydrogenase by replacing the serine hydrogen-bond donor with threonine and replacing the catalytic Zn2+ with Co2+. Based on the electric field enhancement, we make a quantitative prediction of rate acceleration-50-fold faster than the wild-type enzyme-which was in close agreement with experimental measurements. The effects of the hydrogen bonding and metal coordination, two distinct chemical forces, are described by a unified physical quantity-electric field, which is quantitative, and shown here to be additive and predictive. These results suggest a new design paradigm for both biological and non-biological catalysts.

Uncovering Zn2+ as a cofactor of FAD-dependent Pseudomonas aeruginosa PAO1 d-2-hydroxyglutarate dehydrogenase

Pseudomonas aeruginosa couples the oxidation of d-2-hydroxyglutarate (D2HG) to l-serine biosynthesis for survival, using d-2-hydroxyglutarate dehydrogenase from P. aeruginosa (PaD2HGDH). Knockout of PaD2HGDH impedes P. aeruginosa growth, making PaD2HGDH a potential target for therapeutics. Previous studies showed that the enzyme's activity increased with Zn2+, Co2+, or Mn2+ but did not establish the enzyme's metal composition and whether the metal is an activator or a required cofactor for the enzyme, which we addressed in this study. Comparable to the human enzyme, PaD2HGDH showed only 15% flavin reduction with D2HG or d-malate. Upon purifying PaD2HGDH with 1 mM Zn2+, the Zn2+:protein stoichiometry was 2:1, yielding an enzyme with ∼40 s-1kcat for d-malate. Treatment with 1 mM EDTA decreased the Zn2+:protein ratio to 1:1 without changing the kinetic parameters with d-malate. We observed complete enzyme inactivation for the metalloapoenzyme with 100 mM EDTA treatment, suggesting that Zn2+ is essential for PaD2HGDH activity. The presence of Zn2+ increased the flavin N3 atom pKa value to 11.9, decreased the flavin ε450 at pH 7.4 from 13.5 to 11.8 mM-1 cm-1, and yielded a charged transfer complex with a broad absorbance band >550 nm, consistent with a Zn2+-hydrate species altering the electronic properties of the enzyme-bound FAD. The exogenous addition of Zn2+, Co2+, Cd2+, Mn2+, or Ni2+ to the metalloapoenzyme reactivated the enzyme in a sigmoidal pattern, consistent with an induced fit rapid-rearrangement mechanism. Collectively, our data demonstrate that PaD2HGDH is a Zn2+-dependent metallo flavoprotein, which requires Zn2+ as an essential cofactor for enzyme activity.

Inversion of substrate stereoselectivity of horse liver alcohol dehydrogenase by substitutions of Ser-48 and Phe-93

The substrate specificities of alcohol dehydrogenases (ADH) are of continuing interest for understanding the physiological functions of these enzymes. Ser-48 and Phe-93 have been identified as important residues in the substrate binding sites of ADHs, but more comprehensive structural and kinetic studies are required. The S48T substitution in horse ADH1E has small effects on kinetic constants and catalytic efficiency (V/K_m) with ethanol, but decreases activity with benzyl alcohol and affinity for 2,2,2-trifluoroethanol (TFE) and 2,3,4,5,6-pentafluorobenzyl alcohol (PFB). Nevertheless, atomic resolution crystal structures of the S48T enzyme complexed with NAD⁺ and TFE or PFB are very similar to the structures for the wild-type enzyme. (The S48A substitution greatly diminishes catalytic activity.) The F93A substitution significantly decreases catalytic efficiency (V/K_m) for ethanol and acetaldehyde while increasing activity for larger secondary alcohols and the enantioselectivity for the R-isomer relative to the S-isomer of 2-alcohols. The doubly substituted S48T/F93A enzyme has kinetic constants for primary and secondary alcohols similar to those for the F93A enzyme, but the effect of the S48T substitution is to decrease V/K_m for (S)-2-alcohols without changing V/K_m for (R)-2-alcohols. Thus, the S48T/F93A substitutions invert the enantioselectivity for alcohol oxidation, increasing the R/S ratio by 10, 590, and 200-fold for 2-butanol, 2-octanol, and sec-phenethyl alcohol, respectively. Transient kinetic studies and simulations of the ordered bi bi mechanism for the oxidation of the 2-butanols by the S48T/F93A ADH show that the rate of hydride transfer is increased about 7-fold for both isomers (relative to wild-type enzyme) and that the inversion of enantioselectivity is due to more productive binding for (R)-2-butanol than for (S)-2-butanol in the ternary complex. Molecular modeling suggests that both of the sec-phenethyl alcohols could bind to the enzyme and that dynamics must affect the rates of catalysis.

Cloning, functional expression and characterization of a bifunctional 3-hydroxybutanal dehydrogenase /reductase involved in acetone metabolism by Desulfococcus biacutus

Background

The strictly anaerobic, sulfate-reducing bacterium Desulfococcus biacutus can utilize acetone as sole carbon and energy source for growth. Whereas in aerobic and nitrate-reducing bacteria acetone is activated by carboxylation with CO₂ to acetoacetate, D. biacutus involves CO as a cosubstrate for acetone activation through a different, so far unknown pathway. Proteomic studies indicated that, among others, a predicted medium-chain dehydrogenase/reductase (MDR) superfamily, zinc-dependent alcohol dehydrogenase (locus tag DebiaDRAFT_04514) is specifically and highly produced during growth with acetone.

Results

The MDR gene DebiaDRAFT_04514 was cloned and overexpressed in E. coli. The purified recombinant protein required zinc as cofactor, and accepted NADH/NAD⁺ but not NADPH/NADP⁺ as electron donor/acceptor. The pH optimum was at pH 8, and the temperature optimum at 45 °C. Highest specific activities were observed for reduction of C₃ - C₅-aldehydes with NADH, such as propanal to propanol (380 ± 15 mU mg⁻¹ protein), butanal to butanol (300 ± 24 mU mg⁻¹), and 3-hydroxybutanal to 1,3-butanediol (248 ± 60 mU mg⁻¹), however, the enzyme also oxidized 3-hydroxybutanal with NAD⁺ to acetoacetaldehyde (83 ± 18 mU mg⁻¹).

Conclusion

The enzyme might play a key role in acetone degradation by D. biacutus, for example as a bifunctional 3-hydroxybutanal dehydrogenase/reductase. Its recombinant production may represent an important step in the elucidation of the complete degradation pathway.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0899-9) contains supplementary material, which is available to authorized users.

The application of reaction engineering to biocatalysis

Biocatalysis is a growing area of synthetic and process chemistry with the ability to deliver not only improved processes for the synthesis of existing compounds, but also new routes to new compounds.

Exploiting Enzymatic Dynamic Reductive Kinetic Resolution (DYRKR) in Stereocontrolled Synthesis

Over the past two decades, the domains of both frontline synthetic organic chemistry and process chemistry and have seen an increase in crosstalk between asymmetric organic/organometallic approaches and enzymatic approaches to stereocontrolled synthesis. This review highlights the particularly auspicious role for dehydrogenase enzymes in this endeavor, with a focus on dynamic reductive kinetic resolutions (DYRKR) to "deracemize" building blocks, often setting two stereocenters in so doing. The scope and limitations of such dehydrogenase-mediated processes are overviewed, as are future possibilities for the evolution of enzymatic DYRKR.

Modulating the catalytic activity and the substrate specificity of alcohol dehydrogenases using cyclic ethers

The catalytic activity of Thermoanaerobacter brockii alcohol dehydrogenase (Tbadh) is increased by the addition of 1,3-dioxolane, although it is inhibited by the addition of tetrahydrofuran .

Regeneration of nicotinamide coenzymes: principles and applications for the synthesis of chiral compounds

Dehydrogenases which depend on nicotinamide coenzymes are of increasing interest for the preparation of chiral compounds, either by reduction of a prochiral precursor or by oxidative resolution of their racemate. The regeneration of oxidized and reduced nicotinamide cofactors is a very crucial step because the use of these cofactors in stoichiometric amounts is too expensive for application. There are several possibilities to regenerate nicotinamide cofactors: established methods such as formate/formate dehydrogenase (FDH) for the regeneration of NADH, recently developed electrochemical methods based on new mediator structures, or the application of gene cloning methods for the construction of "designed" cells by heterologous expression of appropriate genes.A very promising approach is enzymatic cofactor regeneration. Only a few enzymes are suitable for the regeneration of oxidized nicotinamide cofactors. Glutamate dehydrogenase can be used for the oxidation of NADH as well as NADPH while L: -lactate dehydrogenase is able to oxidize NADH only. The reduction of NAD(+) is carried out by formate and FDH. Glucose-6-phosphate dehydrogenase and glucose dehydrogenase are able to reduce both NAD(+) and NADP(+). Alcohol dehydrogenases (ADHs) are either NAD(+)- or NADP(+)-specific. ADH from horse liver, for example, reduces NAD(+) while ADHs from Lactobacillus strains catalyze the reduction of NADP(+). These enzymes can be applied by their inclusion in whole cell biotransformations with an NAD(P)(+)-dependent primary reaction to achieve in situ the regeneration of the consumed cofactor.Another efficient method for the regeneration of nicotinamide cofactors is the electrochemical approach. Cofactors can be regenerated directly, for example at a carbon anode, or indirectly involving mediators such as redox catalysts based on transition-metal complexes.An increasing number of examples in technical scale applications are known where nicotinamide dependent enzymes were used together with cofactor regenerating enzymes.

Improvement of the stability of alcohol dehydrogenase by covalent immobilization on glyoxyl-agarose

Immobilization of alcohol dehydrogenase (ADH) from Horse Liver inside porous supports promotes a dramatic stabilization of the enzyme against inactivation by air bubbles in stirred tank reactors. Moreover, immobilization of ADH on glyoxyl-agarose promotes additional stabilization against any distorting agent (pH, temperature, organic solvents, etc.). Stabilization is higher when using highly activated supports, they are able to immobilize both subunits of the enzyme. The best glyoxyl derivatives are much more stable than conventional ADH derivatives (e.g., immobilized on BrCN activated agarose). For example, glyoxyl immobilized ADH preserved full activity after incubation at pH 5.0 for 20h at room temperature and conventional derivatives (as well as the soluble enzyme) preserved less than 50% of activity after incubation under the same conditions. Moreover, glyoxyl derivatives are more than 10 times more stable than BrCN derivatives when incubated in 50% acetone at pH 7.0. Multipoint covalent immobilization, in addition to multisubunit immobilization, seems to play an important stabilizing role against distorting agents. In spite of these interesting stabilization factors, immobilization hardly promotes losses of catalytic activity (keeping values near to 90%). This immobilized preparation is able to keep good activity using dextran-NAD(+). In this way, ADH glyoxyl immobilized preparation seems to be suitable to be used as cofactor-recycling enzyme-system in interesting NAD(+)-mediated oxidation processes, catalyzed by other immobilized dehydrogenases in stirred tank reactors.

Higher metal-ligand coordination in the catalytic site of cobalt-substituted Thermoanaerobacter brockii alcohol dehydrogenase lowers the barrier for enzyme catalysis

Thermoanaerobacter brockii alcohol dehydrogenase (TbADH) is a zinc-dependent NADP(+)/H-linked class enzyme that reversibly catalyzes the oxidation of secondary alcohols to their corresponding ketones. Cobalt substitution studies of other members of the alcohol dehydrogenase (ADH) family showed that the cobalt-containing ADHs have a similar active site structure but slightly decreased activity compared to wild-type zinc ADHs. In contrast, the cobalt-substituted TbADH (Co-TbADH) exhibits an increase in specific activity compared to the native enzyme [Bogin, O., Peretz, M., and Burstein, Y. (1997) Protein Sci. 6, 450-458]. However, the structural basis underlying this behavior is not yet clear. To shed more light on this issue, we studied the local structure and electronics at the catalytic metal site in Co-TbADH by combining X-ray absorption (XAS) and quantum chemical calculations. Importantly, we show that the first metal-ligand coordination shell of Co-TbADH is distorted compared to its native tetrahedral coordination shell and forms an octahedral structure. This is mediated presumably by the addition of two water molecules and results in more positively charged catalytic metal ions. Recently, we have shown that the metal-ligand coordination number of the zinc ion in TbADH changes dynamically during substrate turnover. These structural changes are associated with a higher coordination number of the native catalytic zinc ion and the consequent buildup of a positive charge. Here we propose that the accumulation of a higher coordination number and positive charge at the catalytic metal ion in TbADH stabilizes the structure of the catalytic transition state and hence lowers the barrier for enzyme catalysis.

Alanine Racemase Free Energy Profiles from Global Analyses of Progress Curves

Free energy profiles for alanine racemase from Bacillus stearothermophilus have been determined at pH 6.9 and 8.9 from global analysis of racemization progress curves. This required a careful statistical design due to the problems in finding the global minimum in mean square for a system with eight adjustable parameters (i.e., the eight rate constants that describe the stepwise chemical mechanism). The free energy profiles obtained through these procedures are supported by independent experimental evidence: (1). steady-state kinetic constants, (2). solvent viscosity dependence, (3). spectral analysis of reaction intermediates, (4). equilibrium overshoots for progress curves measured in D(2)O, and (5). the magnitudes of calculated intrinsic kinetic isotope effects. The free energy profiles for the enzyme are compared to those of the uncatalyzed and the PLP catalyzed reactions. At pH 6.9, PLP lowers the free energy of activation for deprotonation by 8.4 kcal/mol, while the inclusion of apoenzyme along with PLP additionally lowers it by 11 kcal/mol.

In vitro co-metabolism of ethanol and cyclic ketones

The paper presents the results of studies concerning the effects of four cyclic ketones, i.e. cyclopentanone (Pen), cyclohexanone (Hex), cycloheptanone (Hep) and cyclooctanone (Oct) on metabolism of ethanol (EtOH) in vitro. The fraction S9 (supernatant with the removed mitochondria) was used obtained from homogenized rat livers. An increase in reduced nicotinamide adenine dinucleotide (NADH) was examined spectrophotometrically (at 340 nm) while the substrate disappearance and metabolite increase were determined using head-space gas chromatography. The addition of cyclic ketones statistically significantly affected a decrease in the A(340) level, particularly during co-metabolism of EtOH and Hex. The EtOH loss was significantly higher (than the loss observed during metabolism of EtOH alone) only in EtOH-Hex and EtOH-Hep systems, which may be explained by the fact that reoxidation of NADH to NAD+ is quicker in these systems than dissociation of the alcohol dehydrogenase (ADH)-NADH complex.

Structural basis for substrate specificity differences of horse liver alcohol dehydrogenase isozymes

A structure determination in combination with a kinetic study of the steroid converting isozyme of horse liver alcohol dehydrogenase, SS-ADH, is presented. Kinetic parameters for the substrates, 5beta-androstane-3beta,17beta-ol, 5beta-androstane-17beta-ol-3-one, ethanol, and various secondary alcohols and the corresponding ketones are compared for the SS- and EE-isozymes which differ by nine amino acid substitutions and one deletion. Differences in substrate specificity and stereoselectivity are explained on the basis of individual kinetic rate constants for the underlying ordered bi-bi mechanism. SS-ADH was crystallized in complex with 3alpha,7alpha,12alpha-trihydroxy-5beta-cholan -24-acid (cholic acid) and NAD(+), but microspectrophotometric analysis of single crystals proved it to be a mixed complex containing 60-70% NAD(+) and 30-40% NADH. The crystals belong to the space group P2(1) with cell dimensions a = 55.0 A, b = 73.2 A, c = 92.5 A, and beta = 102.5 degrees. A 98% complete data set to 1.54-A resolution was collected at 100 K using synchrotron radiation. The structure was solved by the molecular replacement method utilizing EE-ADH as the search model. The major structural difference between the isozymes is a widening of the substrate channel. The largest shifts in C(alpha) carbon positions (about 5 A) are observed in the loop region, in which a deletion of Asp115 is found in the SS isozyme. SS-ADH easily accommodates cholic acid, whereas steroid substrates of similar bulkiness would not fit into the EE-ADH substrate site. In the ternary complex with NAD(+)/NADH, we find that the carboxyl group of cholic acid ligates to the active site zinc ion, which probably contributes to the strong binding in the ternary NAD(+) complex.

Stereospecificity of hydrogen transfer by the NAD(+)-linked alcohol dehydrogenase from the Antarctic psychrophile Moraxella sp. TAE123

Investigation of the stereochemistry of the hydride transfer in reactions catalyzed by the recently isolated NAD(+)-linked alcohol dehydrogenase from the Antarctic psychrophile Moraxella sp. TAE123 was accomplished by using 1H NMR spectroscopy of the deuterated coenzyme. It was found that this new psychrophilic enzyme is a type A dehydrogenase. Moraxella sp. ADH reduces stereospecifically 2-butanone to produce (S)-2-butanol.

Enzymatic reaction rate limits with constraints on equilibrium constants and experimental parameters

A general methodology is presented for estimating maximum rates of enzymatic reactions based on general characteristics of enzymatic reaction mechanisms, kinetic limits and thermodynamics. The useful range of experimentally derived kinetic parameters can also be extended by the methodology. The methodology divides the reaction mechanism into physical and chemical steps. Maximum rates that comply with kinetic and thermodynamic constraints are calculated by setting the physical rate constants to their diffusion limits and optimising the chemical rate constants subject to constraints of the reaction mechanism and overall equilibrium constant. Rate estimates from this methodology can be subject to additional constraints from experimental data, and thus conform to the distinctive features of the enzymatic reaction. The methodology is demonstrated using a reversible enzymatic reaction model involving ordered binding of two reactants and ordered release of two products (bi-bi mechanism). Numerical results are shown for alcohol dehydrogenase (EC 1.1.1.1), which has a bi-bi mechanism. Pyrophosphatase (EC 3.6.1.1) with a uni-bi mechanism and triosephosphate isomerase (EC 5.3.1.1) with a uni-uni mechanism are also examined.

Electrostatic effects in the kinetics of coenzyme binding to isozymes of alcohol dehydrogenase from horse liver

The kinetic mechanism for the binding of NAD+ and NADH to the EE and SS isozymes of alcohol dehydrogenase (LADH) was studied between pH 7 and pH 10 by monitoring the quenching of tryptophan fluorescence. A consistent interpretation of all data was only possible by introducing a two-step binding mechanism. The first binding step is related to docking of the adenosine part of the coenzymes and the subsequent isomerization to the binding of the nicotinamide part. At high NADH concentrations an additional slow isomerization was identified as a conformational transition of the protein. A pH dependence for NADH binding is observed which is restricted to changes in the binding kinetics of the adenosine moiety going from pH 7 to pH 10, a tendency which is similar also for NAD+. This is attributed to pH-dependent variations in electrostatic attractions acting as a steering force of the docking process. The nicotinamide docking of NADH is equally fast for both isozymes and pH-independent over the measured range, whereas this docking equilibrium for NAD+ is pH-dependent for EE- and SS-LADH alike and the rate of association comparable. Presumably, a GluEE-366-LysSS substitution results in a stronger binding and faster association of both oxidized and reduced cofactor to the SS isozyme. A structural proof is presented for coenzyme-competitive binding of a sulfate ion, resulting in electrostatic shielding.

The enantiomeric purity of alcohols formed by enzymatic reduction of ketones can be improved by optimisation of the temperature and by using a high co-substrate concentration

The stereoselective reduction of ketones by alcohol dehydrogenase from Thermoanaerobium brockii was studied in organic reaction media. 2-Propanol was used as co-substrate to regenerate the coenzyme NADPH. The enantiomeric excess of the alcohol formed from the ketone decreased during the course of the reaction (from 53 to 0% e.e. in the formation of (R)-2-butanol). This was interpreted as being due to the reversibility of all the reactions involved. By using a large excess of 2-propanol this effect was suppressed. In the reduction of 2-butanone to (R)-2-butanol, the enantiomeric excess increased with increasing temperature, but in the reduction of 2-pentanone to (S)-2-pentanol the enantiomeric excess decreased with increasing temperature. The data were evaluated in terms of free energy of activation of the reaction pathways leading to the different possible products.

Purification and characterization of the alcohol dehydrogenase from a novel strain of Bacillus stearothermophilus growing at 70 degrees C

The biocatalysts isolated from thermophilic microorganisms are the object of ever-growing scientific interest for (i) the comprehension of the molecular basis of their thermal tolerance, and (ii) their use in different bio-industrial fields. Here we report the purification and characterization of an alcohol dehydrogenase (designated ADH-hT) from the novel strain LLD-R of Bacillus stearothermophilus which grows at 70 degrees C. ADH-hT was obtained in pure form by anion exchange chromatography and two affinity chromatographies, with a final yield of about 30%. ADH-hT was found to be a tetramer of 37 kDa-subunits, and to have a pI of 4.9. ADH-hT displayed a broad substrate specificity; its activity was highest for aldehydes, and decreased progressively for alcohols and ketones. ADH-hT was endowed with catalytic activity and resistance in the presence of several denaturing agents (organic solvents, detergents, chaotropic agents). ADH-hT shared with ADH 1503 (the alcohol dehydrogenase from B. stearothermophilus strain NCA 1503 which grows at 55 degrees C) the optimal temperature of 65 degrees C, but it was more resistant than ADH 1503 towards heating. In conclusion, due to its stability and broad substrate specificity ADH-hT could be utilized in bio-industrial processes. Furthermore, we believe that ADH-hT could represent a good model system for studying the mechanism(s) which proteins exploit to gain heat resistance.

Preparation and characterization of isozymes and isoforms of horse liver alcohol dehydrogenase

The procedure described allows the simultaneous large-scale preparation of the three main isozymes (EE, ES, SS) of alcohol dehydrogenase from horse liver (HLADH) and their subfractions using heat denaturation, ammonium sulfate precipitation, DEAE and CM ion-exchange chromatography as well as AMP-Sepharose affinity chromatography. Typical yields that can be obtained within three weeks are 1.5-2.5 g of EE-HLADH, 300-800 mg of ES-HLADH, 20-400 mg of SS-HLADH and 50-100 mg of EE-HLADH isoforms from 5 kg of horse liver. The EE-HLADH isoform prepared has a pI of 7.8, which is 0.3 pH units lower as compared to the main fraction; the zinc content and number of free sulfhydryl groups are unchanged but matrix-assisted laser desorption ionization mass spectrometry resulted in a molecular mass difference of + 130 to 165 relative molecular mass. From a sugar determination and comparison of its pI with an artificial glycosylation product of the EE-HLADH isozyme we concluded that the isoforms of HLADH are non-enzymatic glycosylation products which have been described to occur during protein aging.

Crystallographic studies of isosteric NAD analogues bound to alcohol dehydrogenase: specificity and substrate binding in two ternary complexes

CNAD (5-beta-D-ribofuranosylnicotinamide adenine dinucleotide) is an isosteric C-glycosidic analogue of NAD(H) containing a neutral pyridine ring. CPAD (5-beta-D-ribofuranosylpicolinamide adenine dinucleotide) is a closely related pyridine-containing analogue with the pyridine nitrogen on the opposite side of the ring. CNAD is a potent and specific inhibitor of horse liver alcohol dehydrogenase (LADH), binding with a dissociation constant in the nanomolar range. CPAD binds LADH with an affinity comparable to that of NAD. Crystal structures of CNAD and CPAD bound to LADH are presented at 2.4 and 2.7 A, respectively. The two complexes are isomorphous, crystallizing in the triclinic system with cell dimensions different from those seen in previous ternary LADH complexes. Structures were solved using the molecular replacement method and refined to crystallographic R values of 18% (CNAD) and 17% (CPAD). Both inhibitors bind to the "closed" form of LADH in the normal cofactor-binding cleft. The conformation of LADH-bound CPAD closely mimics that of LADH-bound NAD(H). The data suggest that alcohol substrate binds directly to the catalytic zinc atom. In the CNAD complex, the pyridine nitrogen replaces alcohol as the fourth coordination ligand to the active site zinc atom, while all other polar interactions remain the same as those of bound NAD(H). The zinc-nitrogen ligand explains the high affinity of CNAD for LADH.

A few amino acid substitutions are responsible for the higher thermostability of a novel NAD(+)-dependent bacillar alcohol dehydrogenase

The gene adh-hT encoding a thermostable and thermophilic NAD(+)-dependent alcohol dehydrogenase (ADH) from the novel and more thermophilic Bacillus stearothermophilus LLD-R strain was cloned and its nucleotide sequence determined. The deduced protein sequence shows remarkable amino acid substitutions when compared to the sequence of the protein isolated from strain NCA1503 and significant similarity with the highly thermostable ADH from the thermoacidophilic archaebacterium Sulfolobus solfataricus. The alignment of these sequences led to the identification of three amino acid replacements probably responsible for the higher thermostability of the novel bacillar ADH. Adh-hT gene expression in Escherichia coli, a fast purification procedure and the characterization of the recombinant enzyme are also described.

Alternative pathways and reactions of benzyl alcohol and benzaldehyde with horse liver alcohol dehydrogenase

Liver alcohol dehydrogenase catalyzes the reaction of NAD+ and benzyl alcohol to form NADH and benzaldehyde by a predominantly ordered reaction. However, enzyme-alcohol binary and abortive ternary complexes form at high concentrations of benzyl alcohol, and benzaldehyde is slowly oxidized to benzoic acid. Steady-state and transient kinetic studies, equilibrium spectrophotometric measurements, product analysis, and kinetic simulations provide estimates of rate constants for a complete mechanism with the following reactions: (1) E<-->E-NAD+<-->E-NAD(+)-RCH2OH<-->E-NADH-RCHO<-->E-NADH<-->E ; (2) E-NADH<-->E-NADH-RCH2OH<-->E-RCH2OH<-->E; (3) E-NAD+<-->E-NAD(+)-RCHO-->E- NADH-RCOOH<-->E-NADH. The internal equilibrium constant for hydrogen transfer determined at 30 degrees C and pH 7 is about 5:1 in favor of E-NAD(+)-RCH2OH and has a complex pH dependence. Benzyl alcohol binds weakly to free enzyme (Kd = 7 mM) and significantly decreases the rates of binding of NAD+ and NADH. The reaction of NAD+ and benzyl alcohol is therefore kinetically ordered, not random. High concentrations of benzyl alcohol (> 1 mM) inhibit turnover by formation of the abortive E-NADH-RCH2-OH complex, which dissociates at 0.3 s-1 as compared to 6.3 s-1 for E-NADH. The oxidation of benzaldehyde by E-NAD+ (Km = 15 mM, V/E = 0.4 s-1) is inefficient relative to the oxidation of benzyl alcohol (Km = 28 microM, V/E = 3.1 s-1) and leads to a dismutation (2RCHO-->RCH2OH + RCOOH) as E-NADH reduces benzaldehyde. The results provide a description of final product distributions for the alternative reactions catalyzed by the multifunctional enzyme.